The present invention relates to synthesis and process conditions for lithium transition metal phosphate having an olivine structure, which is receiving a great deal of attention as a next-generation cathode material for a lithium ion secondary battery, e.g. LiMPO4 (M=Fe, Mn, Co, Ni, Ti, Cu, or any combination thereof, and hereinafter M is referred to as this.), and applications thereof. More specifically, the present invention relates to a process for preparing a nanoparticle powder of lithium transition metal phosphate, involving synthesis of lithium transition metal phosphate (LiMPO4) (M=Fe, Mn, Co, Ni, Ti, Cu or any combination thereof) into a nanoparticle powder having a particle size of less than 100 nm to thereby significantly reduce a diffusion distance of lithium ions within particles, which consequently results in full exploitation of a capacity of an electrode material corresponding up to a theoretical capacity thereof and formation of nano-particles having a high electrical conductivity within a short period of time, and which is also capable of achieving efficient industrial-scale production of a desired compound via a heat treatment at a low temperature of less than 600° C. for a short period of time of less than 4 hours while overcoming a shortcoming of a low electrical conductivity, using solid raw materials.
The present invention also relates to a lithium transition metal phosphate with cation exchange defects or cation anti-site defects between lithium and transition metal, which are arranged only in the 1D crystal orientation in the lithium transition metal phosphate (LiMPO4) having an olivine structure in which cations are orderly arranged, and a method of preparing the same.
Most of the oxides having an olivine structure represented by a formula of M′M″ (XO4) (M′ and M″ are each a metal cation including a transition metal, and X is any one selected from the group consisting of P, S, As, Mo, Si, and B) have high melting points and thermally and chemically very stable properties. Therefore, they have been widely used as insulators and refractories requiring excellent thermochemical stability.
Olivine is originally a mineral name of MgFeSiO4, and many oxides having the same crystal structure as this are generally referred to as an olivine-type oxide. Referring to crystallographical properties of olivine-type oxides, an M′ ion is positioned in oxygen octahedral interstitials sharing edges and an M″ ion is positioned in oxygen octahedral interstitials sharing corners.
That is, as shown in FIG. 21, M′ and M″ cations are positioned in different kinds of oxygen octahedra, and sites in each oxygen octahedron are called “M1 site” and “M2 site”. An X atom is relatively small such that it is positioned in tetrahedral sites of carbons. In addition, it forms a polyanion framework of [XO4] as a whole because there are more components of covalent bonding than in M′ and M″ cations. Due to these crystallographic factors, most of the olivine-type oxides are thermally and chemically very stable.
In various oxides of olivine structure, cations are generally positione in the M1 and M2 sites and mixed with each other rather than being orderly arranged. However, there are many cases where the degrees of the two kinds of cations being orderly arranged in each of M1 and M2 sites significantly change as the temperature and pressure change (See C. M. B. Henderson et al., Science, vol. 271, 1713-1715 (1996)).
Therefore, because various physical properties such as electrical conductivity, diffusivity of each ion, plastic deformation of a crystal, etc. can significantly change according to how these two different kinds of cations are distributed in each oxygen octahedron, much research has been conducted on anti-site defects and cation partitioning in oxides of olivine structure (See L M. Hirsch and T. J. Shankland, Geophys. J. Int., vol. 114, 21-35 (1993)).
Among lithium transition metal phosphates (LiMPO4) having the olivine-structure, LiFePO4 and Li(Fe,Mn)PO4 are natural minerals well known as Triphylite. Lithium is positioned in an oxygen octahedral interstitial of the M1 site in LiMPO4, slightly different from other oxides having an olivine structure, and transition metal (M) is positioned in an oxygen octahedral interstitial of the M2 site, showing characteristics of a very ordered olivine structure has. That is, as shown in FIG. 21, the oxygen octahedra including the transition metal (M) are linked in the form of a 1D-chain in LiMPO4, and it is expected that because lithium ion in another oxygen octahedron is sharing edges and orderly arranged in a 1D fashion in the y-axis direction, e.g., b-axis direction, the migration of the lithium ion would be very fast in the y-axis direction (See D. Morgan et al., Electrochem. Solid-State Lett., Vol. 7, p.A30 (2004)).
Further, recent high temperature neutron diffraction experiments and experimental results using a maximum entropy method show that lithium ion moves in a linear fashion in the b-axis direction inside the lattices (See S.-I. Nishimura et al., Nature Mater. Vol. 7, p. 707 (2008)).
Therefore, the use of a lithium transition metal phosphates (LiMPO4) having an olivine structure as a cathode material for the lithium ion battery can permit intercalation or deintercalation of fast lithium ions at room temperature, exhibiting excellent electrochemical performances. However, upon intercalation or deintercalation of lithium ions, an anti-site defect such as the position of transition metal (M) in the M1 site where lithium should be positioned would interfere with the migration of lithium ions in the b-axis direction, and these defects should be controlled for maximal inhibition. In addition, when the anti-site defect is thermodynamically inevitable, a locally 1-D arrangement need to be constructed in order to maximally inhibit those defects which are 3-dimensionally randomly arranged.